An optical interferometer is a device that employs the effects of electromagnetic radiation interference. The electromagnetic radiation that enters the interferometer undergoes multiple reflections and the interference of the electromagnetic radiation emerging from the interferometer during each “bounce” causes a modulation in the transmitted and reflected beams. The interference of waves is the process whereby two or more waves of the same frequency or wavelength combine to form a wave whose amplitude is the sum of the amplitudes of the interfering waves. Constructive and destructive interference occurs based on the angle of the beam, the optical thickness of the interferometer, and the wavelength. The transmission spectrum of the interferometer imaging system displays a series of peaks where constructive interference occurs.
Applications in which interferometers are used as a tool include metrology, spectroscopy, and astronomy. These applications require measurement of small displacements, refractive index changes, and surface irregularities. In addition, these functions require precise measurements of wavelength, the measurement of very small distances and thicknesses, the detailed study of the hyperfine structure of spectrum lines, the precise determination of refractive indices, and, in astronomy, the measurement of binary-star separations and the diameters of stars. Optical interferometers are based on both two-beam interference and multiple-beam interference.
A typical Fabry-Perot interferometer comprises a pair of substantially parallel reflective surfaces, or two parallel highly reflecting mirrors, that are spaced apart to define an optical gap. In some Fabry-Perot interferometers, at least one of the surfaces is movable relative to the other in order to vary the size of the optical gap. In other Fabry-Perot interferometers, the optical gap is fixed, and the optical path length may be varied by tilting the interferometer or varying the air pressure. In use, electromagnetic radiation comprising a number of different wavelengths impinges on the interferometer and passes into the optical gap and is then reflected between the two reflective surfaces. Constructive and destructive interference occurs, leading to certain well-defined wavelengths being transmitted through the interferometer while the remaining wavelengths are not transmitted. In typical Fabry-Perot interferometers, a series of well-defined transmission peaks are obtained corresponding to wavelengths that are transmitted, the wavelengths at which the peaks are situated being adjustable by varying the width of the optical gap. The transmission spectrum as a function of wavelength exhibits peaks of large transmission corresponding to resonances of the interferometer. As the reflectivity of the mirrors is increased, the modulation peaks become sharper and decrease in width.
In traditional Fabry-Perot interferometers, it is important that the reflective surfaces of the interferometer are as parallel as possible in order to minimize distortions that can degrade image or electromagnetic radiation quality. For example, a traditional Fabry-Perot interferometer may require a degree of parallelism of less than ¼ of the wavelength of the source of electromagnetic radiation. This requirement limits the choice of materials as well as the size of the imaging system.
A typical Fabry-Perot interferometer is spatially separated from the focal plane, and the system requires additional imaging optics between the interferometer and the focal plane. Spectral imaging systems for thermal infrared applications, including Fabry-Perot interferometers, are typically inherently large in size, weight, power, and cost compared to spectral instruments for shorter optical wavelengths. The signal levels are generally low due to the narrow spectral bandwidth of individual channels, and thus the thermal self emission of the optics must be reduced to yield acceptable sensor noise performance. This frequently leads to the need for cryogenic cooling of most or all of the imaging optics. Cryogenic cooling requires a Dewar large enough to hold a spectrometer or interferometer, and a cooler large enough to cool this considerable thermal mass in an acceptable period of time.